Zircon refractory bodies are often used in glass production due to the superior corrosion resistance of that material.
Generally speaking, glass corrosion resistance of the refractory is enhanced by increasing density and concentration of the zircon to eliminate pores which may permit melted glass or slag intrusion and to eliminate other refractory components having less glass corrosion resistance than the zircon. The pores and other components each provide potential sites for corrosion and/or erosion to begin.
Densification of zircon has been obtained by sintering a mixture of zircon (ZrSiO.sub.4) with titania (TiO.sub.2), iron oxide (FeO) and/or other zircon grain growth enhancing composition(s). When fired at a sufficiently high temperature, between about 1500.degree. C. and 1600.degree. C., some of the individual zircon crystals grow in size by the absorption of other zircon crystals, while the bulk volume and porosity of the material decrease and the bulk density of the material increases. Pure zircon refractories fired without a densifying agent like titania have a maximum bulk density of only about 245 lbs/ft.sup.3. Bulk densities of up to 270 lbs/ft.sup.3 and more have been achieved using titania as a densifying agent.
The densification of the zircon also appears to be directly proportional to the amount of titania present. As little as about 0.6 or 0.7 percent by weight titania may be sufficient for maximum densification of zircon. However, because theoretically uniform distribution cannot be achieved in practice, about one percent by weight is typically added to zircon for optimum densification. Some densification can be observed with as little as about 0.1 percent by weight titania addition. Excess titania may remain in particle form, be reduced to metallic titanium or possibly combine with other compounds which may be present during sintering.
The term "sinterable components" is used to refer to metals, metallic oxides, glasses and other materials in a sinterable mix or green shape which remain in a refractory in some form after sintering. These are distinguished from water, volatiles and combustibles which evaporate or are driven out of the composition or consumed (oxidized to a gaseous form) before or during the sintering process. The term "refractory components" is used to refer to the "sinterable components" in the form(s) in which they remain in the refractory after sintering.
The purification and densification of zircon to increase corrosion resistance typically reduces that material's resistance to thermal shock damage. Thermal shock damage is physical damage such as spalling, cracking and/or fracturing resulting from rapid and/or extreme temperature changes.
Normally, thermal shock damage resistance of dense ceramic bodies can be improved to a certain degree by various means, particularly by using coarse aggregates. Other means include increased porosity (open or closed), providing heterogeneous particle densities or chemically changing the base material in the matrix by forming a solid solution of it with another material.
The thermal shock damage resistance of densified zircon has been heretofore improved by the addition of coarse aggregates, namely densified zircon grog (prereacted zircon). Dense zircon blocks have been produced this way for use in or in connection with glass furnaces, for example as furnace linings and other glass and slag contact applications such as distribution channels and retainers for platinum bushings used for forming glass fibers. Such zircon refractories are used particularly in the production of textile (E) glass fiber, borosilicate (e.g. Pyrex.RTM.) glasses and certain other specialty glasses which are considered especially corrosive. Porous, undensified ("pressed brick") zircon refractories have also found use in non-glass contact structures above the tanks of such furnaces as zircon resists alkali vapors generated by these processes.
In using coarse aggregates to enhance thermal shock resistance in refractories one balances improving thermal shock damage resistance to achieve a minimum acceptable service level with avoiding diminished long-term corrosion/erosion ("wear") resistance. It should be noted with respect to the latter characteristic that increasing the content of coarse aggregate may also increase the likelihood of wear and even damage due to stoning.
To reduce the likelihood of damage from thermal shock in such prior, densified zircon refractories used, for example, as glass furnace linings, furnace operators have had to carefully control and modify their operating procedures, for example, by providing extremely slow furnace heat-up and cool-down rates, using pressurized heat, etc. It was not uncommon for prior densified zircon refractory blocks forming the lining of such glass melting furnaces to crack during the initial heat-up of the furnace, even when such special precautions were taken. Since such furnaces are intended to be in continuous operation for years, even relatively minor thermal shock damage leading to accelerated localized wear (corrosion/erosion) and early furnace shutdown can have a significant impact on the economics of the furnace.
It would be highly valuable to provide densified zircon refractories having glass corrosion resistance at least comparable to if not greater than that of current densified zircon refractory compositions used in glass furnace applications while providing improved thermal shock resistance.